1. Field of the Invention
The present invention relates to reference-frequency-signal switching circuits, and more particularly, to a reference-frequency-signal switching circuit which selectively outputs either the internal reference-frequency signal generated by an internal reference-frequency-signal oscillator or the reference-frequency signal generated by an external reference-frequency-signal oscillator and which stops the operation of the internal reference-frequency-signal oscillator when the external reference-frequency signal is output.
2. Description of the Related Art
In a cable television (CATV) transmitter, a reference-frequency-signal switching circuit for sending a reference-frequency signal to a reference-frequency-signal source in a head end has been conventionally used.
This reference-frequency-signal switching circuit includes an internal reference-frequency-signal oscillator for generating a reference-frequency signal in its inside. The reference-frequency-signal switching circuit selectively outputs the reference-frequency signal output from the internal reference-frequency-signal oscillator or a reference-frequency signal input externally.
FIG. 2 shows an outlined structure of a conventional reference-frequency-signal switching circuit.
As shown in FIG. 2, the conventional reference-frequency-signal switching circuit is formed of an internal reference-frequency-signal oscillator 21, a reference-frequency-signal input terminal 22, a reference-frequency-signal output terminal 23, a detection circuit 24, a relay 25 having one circuit with two contacts, and a power source terminal 26.
The internal reference-frequency-signal oscillator 21 generates an internal reference-frequency signal having a reference frequency, for example, a frequency of 10 MHz. The internal reference-frequency-signal oscillator 21 is connected between the power source terminal 26 and the ground, and its output end is connected to one fixed contact of the relay 25. At the reference-frequency-signal input terminal 22, an external reference-frequency signal having the reference frequency, for example, a frequency of 10 MHz, is given externally. The reference-frequency-signal input terminal 22 is connected to the input end of the detection circuit 24 and to the other fixed contact of the relay 25. Either the internal reference-frequency signal or the external reference-frequency signal is selectively sent to the reference-frequency-signal output terminal. The terminal is connected to a movable contact of the relay 25. The detection circuit 24 detects the external reference-frequency signal input to the input end, and generates a DC driving voltage at the output end. Its output end is connected to one end of the driving coil of the relay 25. In the relay 25, the other end of the driving coil is grounded.
The reference-frequency-signal switching circuit having the above structure operates in the following way.
In a period when the external reference-frequency signal is not input to the reference-frequency-signal input terminal 22, since the external reference-frequency signal is not sent to the input end of the detection circuit 24 and the DC driving voltage is not generated at the output end of the detection circuit 24, the driving coil of the relay 25 is not driven and the movable contact of the relay 25 is switched to one fixed contact side as shown by a solid line in FIG. 2. The internal reference-frequency signal generated by the internal reference-frequency-signal oscillator 21 is sent to the reference-frequency-signal output terminal 23 from the output end of the internal reference-frequency-signal oscillator 21 through the switched contact of the relay 25. The internal reference-frequency signal having a frequency of 10 MHz is taken out at the reference-frequency-signal output terminal 23.
In a period when the external reference-frequency signal is input to the reference-frequency-signal input terminal 22, since the external reference-frequency signal is sent to the input end of the detection circuit 24 and the DC driving voltage is generated at the output end of the detection circuit 24, the driving coil of the relay 25 is driven and the movable contact of the relay 25 is switched to the other fixed contact side as shown by a dotted line in FIG. 2. The external reference-frequency signal input to the reference-frequency-signal input terminal 22 is sent to the reference-frequency-signal output terminal 23 through the switched contact of the relay 25. The external reference-frequency signal having a frequency of 10 MHz is taken out at the reference-frequency-signal output terminal 23.
As described above, the conventional reference-frequency-signal switching circuit automatically switches the output of the reference-frequency signal such that the internal reference-frequency signal generated by the internal reference-frequency-signal oscillator 21 is output when the external reference-frequency signal is not input to the reference-frequency-signal input terminal 22 and the external reference-frequency signal is output unmodified when the external reference-frequency signal is input to the reference-frequency-signal input terminal 22.
In the conventional reference-frequency-signal switching circuit, when the input external reference-frequency signal is output unmodified, since the internal reference-frequency-signal oscillator 21 operates to output the internal reference-frequency signal, a part of the internal reference-frequency signal may be combined for some reason with the external reference-frequency signal being output. In this case, although the external reference-frequency signal having a frequency of 10 MHz and the internal reference-frequency signal having a frequency of 10 MHz have the identical frequency, if a frequency difference occurs even slightly between them, a beat happens between the two reference-frequency signals. This beat component reduces the quality of the output reference-frequency signal. In addition, since the internal reference-frequency-signal oscillator 21 is operating unnecessarily, power consumption increases.
In the conventional reference-frequency-signal switching circuit, only when the external reference-frequency signal is input, the contact of the relay 25 is switched and the input external reference-frequency signal is output. Since the external reference-frequency signal is sent or is not sent to the reference-frequency-signal input terminal 22 usually by a manual operation, even if an external signal other than the determined external reference-frequency signal is erroneously input to the reference-frequency-signal input terminal 22, the detection circuit 24 responds to the signal and the contact of the relay 25 is switched to output the signal from the reference-frequency-signal output terminal 23.